Late Cretaceous Gneiss In The Eastern Gangdese Magmatic Arc:Petrology And Geochronology

The Late Cretaceous Lilong batholith in the eastern section of the Gangdese magmatic arc,southern Tibet,constitutes the middle to lower crustal components of the magmatic arc,forming a window for exploring the component,growth and thickening of the magmaic arc crust.In this paper,we conducted a petrological and geochronological study for the orthogneisses from the Lilong batholith in order to reveal their protolith and metamorphic P-T conditions and tectonic significance.The gneisses are mainly composed of plagioclase,K-feldspar,quartz,biotite,epidote,muscovite,and with or without amphibole.Their SiO₂ contents range from 61.94 wt.%to 74.39 wt.%,and alumina saturation indexes?A/CNK?are 0.89¹.03,implying the features of calc-alkaline and weakly peraluminous to peraluminous.They show enrichment of LREE and depletion of HREE in chondrite-normalized REE patterns,and are characterized by enrichment of LILE and depletion of HFSE,implying the arc-related granites.These gneisses underwent medium-pressure amphibolite facies metamorphism and partial melting,with peak metamorphic temperature and pressure conditions of 720⁷50? and 0.5⁰.6 GPa.The zircon in the gneisses consists of magmatic core and metamorphic rim.The zircon core yielded the Late Cretaceous crystallization age of 84⁸1 Ma,and the zircon rim yielded the metamorphic age of79⁷4 Ma.The?_Hf?t?values of zircon magmatic core range from+10.2 to+12.2,with corresponding Hf two-stage model ages ranging from 496 to 376 Ma.The study shows that Late Cretaceous gneisses from the Lilong batholith experienced nearly syn-intrusive medium-pressure metamorphism of amphibolite facies and partly melting,and the Gangdese arc experienced late Cretaceous crustal growth and thickening.